Fix handling of 6-byte codepoints in left_adjust_char_head in CESU-8
	 encoding

	---
	 enc/cesu_8.c                | 23 +++++++++++++++++++----
	 test/ruby/enc/test_cesu8.rb |  4 ++++
	 2 files changed, 23 insertions(+), 4 deletions(-)